Excessive application of animal manure to farmland has caused a critical accumulation of phosphorus (P) in soils. There is a potential risk of P release to the soil solution and subsequent leaching of P into surface and groundwater, especially where seasonal flooding occurs. During flooding, iron can be reduced from ferric (Fe³⁺) to ferrous iron (Fe²⁺). Phosphorus associated with Fe-minerals can thereby be released into the soil solution and become mobile. We hypothesize that during periodic flooding of soils, leaching of P into tile-drains will be significantly increased. We monitored tile drain outflow from a Pangborn muck soil in northwestern Washington State, from April 2004 to February 2005. Two systems were compared--a pasture and a cornfield. In both systems, soil test P suggested an excessive accumulation of P and a critical degree of P saturation. Soil water tension was measured using tensiometers placed at 3 soil depths (25, 50, and 100 cm). Drain flow rates were measured and the drain water was analyzed for P, Fe³⁺, Fe²⁺, pH, and redox potential. Periodically, soil water tension approached zero and the redox potential of the drainage water dropped to 180 mV during winter. We observed a significant correlation between P and Fe²⁺ in drainage water from the pasture, but not for the cornfield. In the pasture system, we conclude that P lost in tile drainage was associated with reductive dissolution of Fe. A different geochemical mechanism led the P release in the cornfield. Because no colloids were detected by light scattering analysis, we speculate that P and iron were transported as ternary complexes with dissolved organic matter (DOM).